The present invention relates generally to control circuits and more particularly to a control circuit for supplying a load with electrical power proportional to the middle valued of three input signals.
In the discipline of process control, it is often desirable to develop an output signal (e.g., a control signal) which represents the middle valued of three input signals. These input signals are commonly feedback signals of a system parameter which is being controlled with the three feedback signals being the result of independent sensing of the same parameter. For example, a parameter closely monitored in a gas turbine is the internal temperature of the combustion chamber. Because of the criticality of this measurement, since excessive temperature or excessive rate of temperature rise can be extremely detrimental to the turbine, it is common to use a plurality of sensors. These sensors provide independent feedback signals which, although the sensors are of the same type, vary due to variances in the sensors themselves, differences in the amount of degradation of the sensors and the circuitry involved and/or other factors such as one of the sensors may fail. It is, therefore, commonly known to consider these sensors in groups of three and to select the sensor providing the middle value of the three as being the "most correct" and to regulate the temperature to that value. In a gas turbine, temperature is largely regulated by controlling the fuel flow to the mid-value.
A variety of mid-value circuits are known. Most of these circuits are either very expensive or have a common active component(s) the failure of which will result in a complete loss of the system. As an example, it is known to have an operational amplifier as the common point with the circuitry associated with the individual feedbacks all feeding the common operational amplifier. It is apparent in this case that should this amplifier fail, the control system will either fail or at least be seriously degraded.